1. Field of the Invention
The present invention relates to a method of forming a gallium nitride crystal which can be used for a semiconductor laser having a short wavelength, a transistor which operates at a high speed, etc.
2. Description of the Related Art
A semiconductor laser is widely used as a light source for reading of and writing in an optical disk. Since a recordable amount of information per unit surface area of an optical disk is inversely proportional to a square of a wavelength of a light source, in order to realize high-density recording, it is essential to shorten the wavelength of the light source. Gallium nitride is a direct transition semiconductor which has a forbidden band of 3.4 eV width, and can create a mixed crystal of aluminum nitride and indium nitride. This makes it easy to form a double heterostructure which is necessary to realize a semiconductor laser, and therefore, much expectation is placed on a possible use of gallium nitride as a laser material which has a short wavelength of around 400 nm.
In a conventional method of forming a gallium nitride crystal, sapphire is used for a substrate. A thin film of aluminum nitride or gallium nitride is formed on a sapphire substrate, and a gallium nitride crystal is formed at a higher temperature than a temperature at which the thin film of aluminum nitride or gallium nitride grows.
Now, the conventional method of forming a gallium nitride crystal will be described.
FIGS. 5A to 5C are cross sectional views showing steps in sequence and describing the conventional method of forming a gallium nitride crystal. In FIGS. 5A to 5C, denoted at 5 is a gallium nitride crystal, denoted at 6 is a substrate of sapphire, for example, and denoted at 7 is a thin film of aluminum nitride, for example.
In this method of forming a gallium nitride crystal, as shown in FIGS. 5A and 5B, by reacting trimethyl aluminum and ammonia at a growth temperature of 550.degree. C., for instance, the aluminum nitride thin film 7 is formed to have a thickness of about 300 .ANG. on the sapphire substrate 6. Following this, the substrate is heated to a temperature of 1,050.degree. C., for example, to react trimethyl gallium and ammonia, whereby the gallium nitride crystal 5 is formed into a thickness of 4 .mu.m as shown in FIG. 5C.
However, in the conventional method of forming a gallium nitride crystal described above, lattice constants along the a-axis of a hexagonal crystal of the sapphire substrate, gallium nitride and aluminum nitride are 4.758 .ANG., 3.189 .ANG.and 3.111 .ANG., respectively. Thus, the lattice constants of the sapphire substrate, the gallium nitride and the aluminum nitride are largely different from each other. Because of this, a stress is created between the sapphire substrate and growing layers of the gallium nitride and the aluminum nitride during growth, which in turn creates a dislocation or a crack in the growing layers.
In the example of the conventional method above, a density of dislocation within the gallium nitride crystal is about 10.sup.9 cm.sup.-2. In general, it is difficult to obtain a gallium nitride crystal which has a smaller density of dislocation than this.
Further, since a cleavage plane orientation of the sapphire substrate and a cleavage plane orientation of gallium nitride are different by 30 degrees, it is not possible to obtain an excellent cleavage plane of gallium nitride. This forces use of other methods except for cleaving, such as dry etching, for obtaining an oscillator which is necessary to realize a semiconductor laser apparatus. Thus, there are a number of difficulties in obtaining an excellent oscillator, in reality.